Abstract

In recent years, materials with porous architecture and high surface area are being developed for numerous potential applications in nanotechnology, catalysis and separation science, molecular sieves, zeolites, hybrid optics, bio ceramics etc. We have synthesized porous wollastonite (CaSiO3) and $Fe^{3+}$ doped (0.5, 1, 1.5, 2, 2.5, 3 and 5 mol%) ceramic powders by a novel low-temperature initiated, self-propagating, gas producing solution combustion process. The ceramic powders have been studied by XRD, TG/DTA, SEM, EDS and EPR spectroscopic techniques. The effect of temperature on crystalline phase formation, amount of porogens and particle size of porous wollastonite has been investigated. The XRD patterns confirm that the ceramic powders undergo a clear single phase formation of \beta -CaSiO3 and \alpha -CaSiO3 at 950°C and 1200°C respectively. These formation temperatures were lower than those using other methods by the conventional solid-state reaction method. It is observed that the average particle size of the annealed wollastonite samples is in the range 29-50 nm. The samples calcined to 950°C has low porosity (17.5 %), however the porosity increases with calcination, and at 1200°C has a large porosity of (31.6 %). The surface areas of as formed, 950°C and 1200°C calcined wollastonite samples were 31.93 m2/g, 0.585m2/g and 3.48 m2/g respectively. The EPR spectra of all the investigated samples exhibit resonance signals characteristic of the $Fe^{3+}$ ions in rhombic and axial symmetry sites. The g values indicate that the paramagnetic ion is in trivalent state and the site symmetry is distorted octahedral. The number of spins participating in resonance (N) and the paramagnetic susceptibilities (\chi) has been evaluated from EPR data as a function of $Fe^{3+}$ content. The effect of alkali ions (Li, Na and K) on the EPR spectra, have also been studied and it is interesting to observe that they exhibit a marked alkali effect. The results obtained from these studies have been discussed in detail.